The present invention is directed to a process for the purification of phosphoric acid. A solvent extraction process is employed to remove ionic metallic impurities, such as Ca.sup.+2, Mg.sup.+2, Al.sup.+3 and Fe.sup.+2,+3, from unpurified phosphoric acid, preferably unpurified wet process phosphoric acid prepared by the dihydrate, the hemihydrate or the anhydrite process. The purified phoshoric acid can then be employed in the production of high quality superphoshoric acid (SPA).
As is known in the art, SPA is produced by conventional techniques of evaporation and/or dehydration of phosphoric acid. The P.sub.2 O.sub.5 content of the SPA so produced is from a minimum of about 65% P.sub.2 O.sub.5 to values over 100% P.sub.2 O.sub.5.
If unpurified phosphoric acid is used in the SPA process, SPA possessing a high viscosity is usually produced. On aging, a plastic-or glass-like material results which is extremely difficult to handle. On the other hand, SPA produced from purified phoshoric acid does not exhibit these deleterious properties.
As shown in "Phosphoric Acid by the Wet Process" by Robert Noyes, Noyes Development Corp., Park Ridge, N.J., 1967, SPA is preferred over phosporic acids containing lesser amounts of P.sub.2 O.sub.5 such as merchant grade phosphoric acid containing about 54% P.sub.2 O.sub.5 because:
1. Higher grade liquid fertilizers can be produced from the higher P.sub.2 O.sub.5 content acid. PA1 2. Higher analysis superphosphates and ammonium phosphates can be produced. PA1 3. SPA carries larger quantities of micronutrients. PA1 4. It's more economical to handle and transport SPA (more pounds of P.sub.2 O.sub.5 per pound of material handled.) PA1 5. SPA produces a better solid fertilizer. PA1 6. A controlled release fertilizer can be produced from SPA.
A solvent extraction process has been described by Williams et al. U.S. Pat. No. 3,694,153 (hereinafter referred to as Williams) for the purification of wet process phosphoric acid. Using liquid water immiscible organic sulfonic acids dissolved in organic solvents, Williams was able to remove ionic metallic impurities from the phosphoric acid. Using 55% acid, Williams removed 18% of the calcium, 22% of the magnesium and 1% of the P.sub.2 O.sub.5. When 28% P.sub.2 O.sub.5 acid was used, approximately 30% of the calcium, 24% of the magnesium and 7% of the P.sub.2 O.sub.5 were removed from the phosphoric acid.